The overall objective of this research proposal is to improve the capacity of stem cells to engraft the megakaryocytic lineage. Ex vivo expansion of bone marrow megakaryocyte progenitors in the presence of thrombopoietin (TPO) was significantly enhanced by a soluble factor present in cultures of confluent marrow stromal cells. We are presently attempting to identify this factor. Addition of interleukin-3, but not stem cell factor, to TPO, enhances the expansion of peripheral blood megakaryocyte progenitors from CD34+ cultures, similarly to bone marrow cells. The evaluation of megakaryocytopoiesis from PIXY321 vs. GM-CSF-mobilized peripheral blood progenitor cells yielded the following results: the expansion efficiency of megakaryocytes (MK), defined as the number of MK produced per seeded CD34+ cells, was greater in PIXY321- than in GM-CSF-mobilized samples. However, since the frequency of CD34+ cells was greater in GM-CSF- than in PIXY321-mobilized samples, there was no significant difference in the overall absolute number of MK produced in PIXY321- and OM- CS F-mobilized samples. These results will be useful for ex vivo expansion of MK to treat post-transplant thrombocytopenia.